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Transient Absorption Spectroscopy of Anatase and
Rutile: the Impact of Morphology and Phase on
Photocatalytic Activity
Xiuli Wang,ab‡ Andreas Kafizas,b‡* Xiaoe Li,b Savio J. A. Moniz,c Philip J. T. Reardon,c Junwang
Tangc Ivan P. Parkind and James R. Durrantb*
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 457 Zhongshan Road,
Dalian, China
b
Department of Chemistry, Imperial College London, South Kensington Campus, London, SW7
2AZ, UK
c
Department of Chemical Engineering, University College London, Torrington Place, London
WC1E 7JE, UK
d
Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ,
UK
‡
These authors contributed equally.
ABSTRACT
1
We employed transient absorption spectroscopy (TAS) to investigate the kinetic dependences of
photocatalysis in anatase and rutile TiO2 films of varying morphology. In mesoporous films,
anatase was ~ 30 times more efficient than rutile in the photocatalytic degradation of an
intelligent ink model system. Independent of phase, up to 100 lower levels of photocatalysis was
found in dense films. Charge carrier lifetimes were probed by TAS on the μs – s timescale. For
both rutile and anatase, recombination was independent of morphology. Rutile exhibited up to 10
times slower recombination kinetics than anatase. Efficient, irreversible hole scavenging by
alcohols was present in mesoporous anatase alone, resulting in the generation of long lived
electrons (τ ≈ 0.7 s) which, upon the addition of the dye reduction target resazurin, enabled
efficient electron transfer (τ ≈ 3 ms). Hole scavenging by alcohols on mesoporous rutile was
substantially less efficient and more reversible than anatase, resulting in only a marginal increase
in electron lifetime. The lower activity of rutile was not due to differences in recombination, but
rather from the deficiency of rutile holes to drive efficient and irreversible alcohol oxidation.
KEYWORDS: Transient absorption spectroscopy, photo-generated carrier lifetime,
photocatalysis, morphology, intelligent ink
INTRODUCTION
TiO2 is the most widely employed material for photocatalytic water splitting and degradation
of organic pollutants. Its importance can be attributed to its unique physicochemical properties. 1–
3
For photocatalytic applications, anatase and rutile are the two most studied TiO2 polymorphs.
Despite possessing the same chemical composition, the different arrangements of TiO6 octahedra
in anatase (4 edge sharing partly distorted octahedra) and rutile (2 edge sharing non-distorted
2
octahedra) result in different physicochemical properties. Aside from phase mixtures and
composites, the anatase form is generally regarded as the more active phase in photocatalysis,1
especially for environmental applications2, despite rutile often showing superior photocatalytic
performance in specific cases, such as water oxidation.4,5 This is further supported by its
commercial application, where anatase is used exclusively in self-cleaning windows (Pilkington
Activ, Saint-Gobain Bioclean, PPG Sunclean) and tiles (TOTO Hydrotect).6 Understanding why
anatase often shows a higher activity compared with rutile in most photocatalytic reactions
remains a key challenge that will support strategies to enhance TiO2 photocatalysis.
Nowadays, most studies of photocatalytic reactions are focused on particulate systems (as
opposed to their dense thin film counterparts) due to the apparent reality that a high surface area
is needed to achieve high quantum yields.7 Based on similar arguments, mesoporous TiO2 films
have been actively developed.8 However, it is the dense form of TiO2 that is found in commercial
self-cleaning products, where efficiency has been compromised for material durability. Although
substantial efforts have been made to control the morphology of TiO2 to improve photocatalysis,
the role of morphology in photocatalysis is not clear from the aspect of kinetics. To understand
this, in this study we will examine the difference in photocatalytic activities for mesoporous and
dense thin films of both pure anatase and rutile phase. We will then compare these differences in
activity with the lifetime and population of photo-generated charges using transient absorption
spectroscopy (TAS).
TAS is a form of laser flash spectroscopy that tracks transient changes in absorption after an
excitation pulse. 9–11 It has been shown that the dynamics specific to photo-generated electron or
hole carriers could be studied in TiO2 by tracking transient absorbance changes at particular
wavelengths.12 This has allowed many research groups to use TAS to understand the dynamics
3
of charge transfer in many photocatalytic processes, such as the reduction of O 2,13 the oxidation
of NO14 and the oxidation of water.15 To date, such studies have largely been limited to anatase,
where no direct comparison of the charge carrier dynamics in anatase and rutile TiO2 has been
made.
This study begins with a comparison of the oxidation of methanol, a widely used hole
scavenger in photocatalysis. Although there have been TAS studies of the hole scavenging effect
of alcohols with anatase,16 to our knowledge, there has been no such study with rutile. This study
is then extended to intelligent ink, previously shown to be an effective model system for
quantifying photocatalytic activity.17,18 The ink consists of three components dissolved in water:
(i) a polymer thickener, (ii) glycerol and (iii) a resazurin redox dye. Intelligent ink is made rapid
acting through the use of a hole scavenger, glycerol. Such alcohols are well known for their hole
scavenging potency and are regularly used to increase the efficiency of photocatalysis, including
water splitting.2 In removing holes, electron-hole recombination is retarded and photo-generated
electrons have more time to take part in reduction processes of the dye, as summarized in
Scheme 1.
Scheme 1. Two-step reduction of resazurin (royal blue) to resorufin (pink) and then dihydroresorufin (colorless).
The focus of this study is to understand how the phase and morphology of TiO2 impacts on the
kinetic competition between electron-hole recombination and their reaction with organics in
solution – enabling better materials design for future optimization of the photocatalytic function.
4
EXPERIMENTAL SECTION
Sample preparation
Mesoporous TiO2 film
A colloidal anatase paste was prepared from the aqueous hydrolysis of titanium isopropoxide,
as described in our previous publications.19 A colloidal rutile paste was prepared from a
commercial powder (Shanghai ST-Nano Science and Technology Co.).20 Mesoporous films were
formed by doctor-blading each colloidal paste onto glass microscope slides, cleaned with acetone
prior to use. The pastes were allowed to air-dry for 30 min before calcination at 450 oC for 30
min.
Dense TiO2 film
Dense anatase thin-films were grown on quartz substrates by chemical vapor deposition from
the reaction of TiCl4 and ethyl acetate vapors carried by N2 gas at 500 oC. Dense rutile films
were made by simply annealing the dense anatase films at 1050 oC in air for 5 hours to ensure
the complete conversion to its thermodynamic phase.21
Physical characterization
Surface microstructures were investigated by scanning electron microscopy (SEM) on a JOEL6301F field emission instrument. X-ray diffraction (XRD) patterns were measured with a
modified Bruker-Axs D8 diffractometer with parallel beam optics equipped with PSD LinxEye
silicon strip detector. Patterns were fit to a Rietveld refined model using GSAS-EXPGUI
software.22 UV-visible transmittance and reflectance spectra were recorded from 250 – 1100 nm
using a Helios double beam instrument equipped with an integrating sphere. Specific surface
area measurements were measured using the Brunauer–Emmett–Teller (BET) method23 in N2
5
using a TriStar 3000 Micromeritics device. Pore diameter distributions were determined using
the Barret-Joyner-Halenda (BJH) method.24 Mesoporous film thickness was measured using an
Alpha 200 Profilometer.
Photocatalytic activity
The photocatalytic test involved spray-coating intelligent ink25 evenly over the surface of the
film.18 The intelligent ink (royal blue in color) consisted of hydroxyl ethyl cellulose polymer
(0.45 g), glycerol (3.0 g) and the redox dye resazurin (40 mg) dissolved to 30 mL in distilled
water. Samples were spray-coated with the ink using an aerosol spray gun (SIP Emerald Spray
Gun, Halfords, Plc.) at an air-pressure feed of 3.5 bar until a blue coating was formed. UVvisible absorption spectra were measured from 450 – 700 nm using a PerkinElmer Lambda 25
device. A soft UVA light source was used as the excitation source for the photocatalytic reaction
(365 nm –Vilber Lourmat 2 x 8W, Figure S1), which was shone from above (i.e. lamp – ink –
sample – substrate). The lamp flux was measured using a UVX-Radiometer equipped with a
UVX-36 sensor (λ = 365 ± 15 nm).
Previous studies have shown the photocatalytic reduction of intelligent ink on anatase TiO2 is
first order, where the rate constant for the reduction of resazurin to resorufin is almost 4 times
larger than the reduction of resorufin to dihydro-resorufin.26 As such, the rate of the first
reduction (Scheme 1) can be determined by assessing the decrease in absorbance at 608 nm
(λmax, resazurin) at early times (i.e. times in which the rate of decrease is linear). The thickness of
the ink coating could be derived using the Beer-Lambert law given the dye was 5.8 mM in
concentration and possesses an extinction coefficient of 43,500 M-1cm-1 at 608 nm. The rate of
decrease at 608 nm could then be converted into a molecular rate. Thus with knowledge of the
6
light flux from the 365 nm excitation source (450 μW.cm-2 ≈ 1.1 x 1015 photons.cm-2.s-1) coupled
with the material’s absorbance at 365 nm the quantum yield (QY) was derived:
𝑄𝑌 (%) =
𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑑𝑒𝑔𝑟𝑎𝑑𝑒𝑑 (𝑐𝑚−2 𝑠 −1 )
× 100
𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 (𝑐𝑚−2 𝑠 −1 )
(1)
Transient absorption spectroscopy
Charge carrier dynamics of TiO2 films were measured using transient absorption spectroscopy
(TAS) from the μs – s timescale at room temperature. The TAS apparatus has been described in
detail elsewhere.27 In brief, a 75 W Xe lamp is used as a probe beam with a monochromator
placed before the sample. The change in transmitted light is measured by a Si PIN photodiode
after a UV laser excitation pulse is applied on the sample using the third harmonic of a Nd: YAG
laser (355 nm, 6 ns pulse width). Reasonably low laser intensities were used (~ 30 - 500 µJ cm-2
pulse-1) with a laser repetition rate of 1 Hz. Each TAS trace is the result of averaging between 50
– 500 scans.
RESULTS
Materials characterization
X-ray diffraction showed that the mesoporous and dense films of anatase (I41/ amd) and rutile
(P42/ mnm) were phase-pure (Figure S2). Scherrer line broadening studies28 showed that the
average width of crystallites were ~ 40 nm in the mesoporous anatase film, ~ 40 nm in the dense
anatase film, ~ 110 nm in the mesoporous rutile film and ~ 80 nm in the dense rutile film. Dense
films showed strong preferred growth effects, akin to epitaxially grown films by CVD.29 No
preferred growth was observed in mesoporous films, typical of particulate growth.
7
The specific surface areas of mesoporous anatase and rutile (Figure S4) as determined by the
Brunauer–Emmett–Teller (BET) method were measured to be 100 m2.g-1 and 72 m2.g-1
respectively. The specific surface areas of our dense films were lower than the detection limits of
the device (< 1 m2.g-1). Barret-Joyner-Halenda (BJH) analysis showed that the average pore
diameters in mesoporous anatase (~ 25– 30 nm) and rutile (~ 24 – 31 nm) were similar. Anatase
was more porous, possessing an average pore volume of 0.82 cm3.g-1 compared with 0.20 cm3.g-1
for rutile. Film topography was investigated by SEM (Figure 1). Mesoporous films were rough
and porous, with anatase possessing primarily spherical particles ~ 20 nm in width and rutile
possessing more fluffy particles that ranged from 100 – 500 nm in width. Dense films were more
compact and flat, with anatase possessing tetragonal domains 100 – 200 nm wide and rutile
possessing larger and more rounded domains 200 – 600 nm wide.
8
Figure 1. SEM images of (a) mesoporous anatase, (b) mesoporous rutile, (c) dense anatase and
(d) dense rutile films.
Photocatalytic activity
The photocatalytic activity of mesoporous anatase and rutile films was examined using the
intelligent ink test as a model system.25 Figure 2a depicts the changes observed by UV-visible
spectroscopy at selected times during the reaction on mesoporous rutile. At 0 s, a spectrum
characteristic of the sole presence of resazurin is observed.30 At the early stages of the reaction,
predominantly resazurin (λmax = 608 nm) is reduced to resorufin (λmax = 575 nm). By 450 s most
of the resazurin was consumed and a maximum concentration of resorufin was reached. By 1000
9
s the resazurin dye was completely reduced and a spectrum characteristic of the sole presence of
resorufin is observed.26 Further irradiation caused the resorufin spectrum to decrease in intensity
as it was reduced to its colorless counterpart dihydro-resorufin.
By plotting the change in absorbance at 608 nm against irradiation time (Figure 2b) one can
track the kinetics of dye reduction from resazurin to resorufin.26 Almost no change was observed
in the blank glass sample, proving dye reduction was due solely to semiconductor mediated
photocatalysis. The photocatalytic conversion of resazurin to resorufin was two orders of
magnitude faster in mesoporous anatase than dense anatase, two orders of magnitude faster than
mesoporous rutile and more than three orders of magnitude faster than dense rutile.
Figure 2. (a) The changing UV-visible absorption spectra during the photocatalysis of intelligent
ink on mesoporous rutile under excitation at 365 nm and (b) the change in normalized
absorbance at 608 nm (λmax resazurin) with time (s, log timescale) during the photocatalysis of
intelligent ink for mesoporous anatase [open circles], mesoporous rutile [black circles], dense
anatase [open squares], dense rutile [black squares] and a blank piece of glass [grey circles].
The quantum yield (QY) for resazurin photo-degradation was determined, taking into account
the differences in film optical densities at the excitation wavelength (365 nm) derived from true
absorbance spectra (Figure S3). Mesoporous anatase and rutile films showed QYs of 62 % and
10
1.9 % respectively. As such, the reaction was ~ 30 times more efficient in mesoporous anatase.
This dramatic difference could not be attributed to the marginal difference in sample surface area
(anatase = 100 m2.g-1 versus rutile = 72 m2.g-1). The dense anatase film showed a QY of 1.2 %
and the dense rutile film showed a QY of 0.09 %. This decrease in efficiency for each phase
relative to mesoporous form was unsurprising as dense films typically possess surface areas < 1
m2.g-1.31 Even though dense samples possessed a far lower surface area, dense anatase possessed
a QY similar to that of mesoporous rutile. Such comparisons show that both phase and surface
area are important if one wishes to produce an efficient TiO2 photocatalyst where in this study:
mesoporous anatase > dense anatase ≈ mesoporous rutile > dense rutile
(2)
Transient absorption spectroscopy
Employing scavengers to identify photo-generated electron and hole spectra
Insight into photo-generated charge carrier events, including phenomena occurring at the
material-reactant interface, can be acquired using transient absorption spectroscopy (TAS).12 In
both anatase and rutile, TAS spectra measured in N2 were composed of broad slopes that showed
stronger absorbance at shorter wavelengths (Figure 3). When in N2 (an inert atmosphere), the
photo-generated electrons and holes in TiO2 recombine non-geminately15 (i.e. no surface
reactions can occur).
11
Figure 3. Transient absorption spectra recorded in N2, a methanol hole scavenger solution or a 2
mM AgNO3(aq) electron scavenger solution for (a) mesoporous anatase and (b) mesoporous rutile
20 µs / 20 ms after 355 nm laser excitation (6 ns pulse width, 1 Hz, 250 μJ.cm-2.pulse-1). Dotted
arrows indicate which y axis to refer to.
To identify the regions where photo-generated electrons and holes were present in TAS spectra,
chemical scavengers were employed. For holes, the well known electron scavenger AgNO3 was
used.32 Similar to previous studies of anatase,33,34 the large rise in absorbance at shorter
wavelengths was attributed to the presence of photo-generated holes. The change in spectral
signature for both anatase and rutile shows how photo-generated electrons are quenched by
AgNO3 on the pre-μs timescale. In the measured wavelength range, hole absorption increased at
shorter wavelengths. For electrons, the well known hole scavenger methanol was used.15 In
anatase, photo-generated holes are completely quenched by methanol on the pre-μs timescale,
whereas in rutile they are only partly quenched (evidenced below). Similar to previous
studies,35,36 a rise in absorbance at longer wavelengths was attributed to the presence of photogenerated electrons (at around 900 nm for both anatase and rutile). It is apparent that the electron
12
signal in rutile is relatively small compared with anatase, most probably due to a lower extinction
coefficient.
Charge recombination in the absence of electron or hole scavengers
Figure 4 shows the transient absorption decays at 900 nm of both mesoporous and dense TiO2
in N2. At this probe wavelength, the photo-induced absorption comes primarily from TiO2
electrons,12 as demonstrated in Figure 3. For anatase, the decays show power law dynamics and
for rutile, the signal decays linearly on a semi-log timescale, consistent with previous reports.37,38
Photo-generated carriers are longer lived in rutile than in anatase. Irrespective of dense or
mesoporous morphology, analogous transient absorption decay kinetics were found in each
phase. This shows how the non-geminate recombination of charges is relatively independent of
topography in TiO2 (i.e. particle size, density, morphology etc) but dependent on crystal
structure. Decays in anatase showed a modest retardation with a decrease in excitation density
(Figure S5a), whereas decays in rutile were almost independent of excitation intensity (Figure
S5b). At the lower excitation densities studied, carrier lifetimes in anatase were more similar to
rutile (Figure S6).
13
Figure 4. Transient absorption decays probed at 900 nm and recorded in an inert N2 atmosphere
for (a) mesoporous and (b) dense TiO2 (normalized to 1 at 10 µs; 355 nm laser excitation, 6 ns
pulse width, 1 Hz, 250 and 300 μJ.cm-2.pulse-1 for mesoporous and dense TiO2 respectively).
Data are shown for anatase (black) and rutile (red) TiO2. Non-normalized decays are shown
inset.
The decay kinetics in methanol and other alcohols
The hole scavenging effect of methanol was studied in both mesoporous and dense films.
Interestingly, methanol (and other alcohols such as isopropanol and glycerol) showed no
noticeable effect on the transient absorption decay dynamics in dense films (Figure S7).
However, substantial changes were observed in mesoporous films (Figures 5 & 6). This clearly
shows how nano-structuring TiO2 enables charge carriers to reach the surface and thus produces
a more active photocatalyst.
Figure 5 shows the effect of methanol on the photo-generated carrier dynamics in mesoporous
anatase. The presence of methanol both increases the initial absorption of mesoporous anatase at
900 nm almost threefold and increases the half-lifetime of the decay from ~ 100 µs to ~ 0.7 s
signifying a reaction with methanol on the pre-μs timescale (Figure 5a). Correspondingly, the
initial absorption at 460 nm decreases to one third of the original signal, with the residual signal
in the presence of methanol decaying with the same dynamics as those at 900 nm, and therefore
assigned to the tail of electron absorption extending to this wavelength.
14
Figure 5. Transient absorption decays recorded in either an inert N2 environment (black lines) or
an N2 purged pure methanol solution (red lines) for mesoporous anatase following a 355 nm
laser excitation pulse (6 ns pulse width, 1 Hz, 250 μJ.cm-2.pulse-1). Samples were probed at (a)
900 nm or (b) 460 nm.
The effect of methanol on photo-generated carrier dynamics in mesoporous rutile was also
studied (Figure 6). Decay kinetics were probed at 900 and 550 nm to monitor changes in photogenerated electrons and holes respectively (see Figure 3). The initial absorption at 550 nm was
reduced by ~ 2/3 in methanol compared with N2 alone. However, unlike anatase, the signal at
550 nm decayed to zero by 1 ms. As the kinetics at 550 nm did not track the kinetics at 900 nm,
they each represent different processes. The decay at 550 nm is attributed to the hole scavenging
of methanol that started on the pre-μs timescale but ended by 1 ms. The decay at 900 nm is
attributed to a partially enhanced electron signal. The decay dynamics at 900 nm was not
significantly prolonged by the addition of methanol until after 1 ms – the same time in which all
holes at 550 nm had been scavenged. Although the addition of methanol results in a scavenging
of approximately 2/3 of rutile holes on the pre-µs timescale, it does not significantly increase the
15
lifetime of photo-generated rutile electrons until 1 ms. Similar decay kinetics were observed in
isopropanol and glycerol (Figure S8).
Figure 6. Transient absorption decays recorded in either an inert N2 environment (black lines) or
an N2 purged pure methanol solution (red lines) for mesoporous rutile following a 355 nm laser
excitation pulse (6 ns pulse width, 1 Hz, 250 μJ.cm-2.pulse-1). Samples were probed at (a) 900 nm
or (b) 550 nm.
Analysis of photocatalytic function
In order to directly correlate our transient absorption data with photocatalytic activity, our
transient absorption studies were extended to our intelligent ink study. In breaking the reaction
down, the different photocatalytic processes that occur within intelligent ink were studied by
TAS (Figures 7 and S9).
16
Figure 7. A transient absorption spectroscopy study of the various photocatalytic processes in
intelligent ink. The normalized transient decays were recorded in air, water, glycerol, resazurin
dye (4 mM) and intelligent ink for (a) mesoporous anatase and (b) mesoporous rutile after backexcitation with a 355 nm laser (6 ns pulse width, 1 Hz, 250 μJ.cm-2.pulse-1) probed at 900 nm
[electron signal].
In mesoporous anatase, the electron decay kinetics in air, water and N2 were indistinguishable,
consistent with previous studies that showed electron transfer to water and O2 are slow (up to
100 μs).39 The addition of glycerol resulted in similarly long lived electrons (τ ≈ 0.2 s) to those
observed with the addition of methanol (Figure 5). In the presence of resazurin dye, but no
glycerol, the electron signal decayed in a similar fashion to its natural decay in air, indicating
electron-hole recombination was faster than dye reduction in the absence of a hole scavenger.
However, in the presence of both glycerol and resazurin, the electron signal showed different
decay kinetics, assigned to the transfer of electrons from mesoporous anatase to the resazurin dye
(τ ≈ 3 ms). This electron transfer is 100 times faster than the electron decay in glycerol alone and
consistent with the high quantum yield observed in our photocatalysis studies (QY = 62 %,
Figure 2). In contrast, all transient decays in mesoporous rutile showed similar kinetics, with
17
only a modest retardation of the transient decay at long times in the presence of glycerol. This
relative independence of the transient decay kinetics in the presence of the intelligent ink clearly
indicates poor hole scavenging by glycerol and poor electron transfer from rutile to resazurin,
which was also consistent with the low quantum yield observed in our photocatalysis studies
(QY = 1.9 %)
DISCUSSION
Charge transfer and decay dynamics of the photo-generated electrons and holes in mesoporous
anatase and rutile were studied by transient absorption spectroscopy (TAS) in dense and
mesoporous, anatase and rutile TiO2. The results were correlated with the photocatalytic activity
of reducing an intelligent ink, which shows that anatase is more efficient than rutile, and
mesoporous films more efficient than dense. The results of these kinetic studies are summarized
in Table 1, and discussed in detail below.
Medium
mesoporous
anatase
mesoporous
rutile
N2
-
+
AgNO3(aq) (2 mM) in
N2
methanol in N2
intelligent ink in air
e- + Ag+,
h+ + H2O/ other
h+ + methanol,
e- + methanol
radicals
h+ + glycerol,
e- + resazurin
Processes
e +h
initial time
from 10
μs
from 10 μs
from 10 μs
from 300 μs
e- (900 nm)
80 μs
completely quenched
0.7 s
10 ms
+
80 μs
0.7 s
completely quenched
completely quenched
-
1 ms
completely quenched
2 ms
transfer not observed
+
1 ms
0.9 s
~ 2/3 quenched
~ 2/3 quenched
h (460 nm)
e (900 nm)
h (550 nm)
Table 1. A summary of the charge transfer/ decay times (t1/2) for mesoporous anatase and rutile
in a range of media studied by transient absorption spectroscopy on the μs – s timescale
Charge recombination in the absence of electron or hole scavengers
18
Our transient absorption data shows that electron-hole charge carrier lifetimes are longer in
rutile than anatase, assigned to a slower non-geminate recombination. These lifetimes are also
invariant in mesoporous and dense microstructures. As such, the higher photocatalytic activity
observed for anatase and mesoporous films could not be assigned to a slower non-geminate
recombination loss.
In anatase, the carrier decay dynamics showed power law, intensity dependent recombination,
characteristic of bi-molecular recombination in the presence of shallow trap states.40 In contrast,
rutile showed linear decay dynamics on log timescale and intensity independent recombination.
The origin of this distinct behavior for rutile is unclear, but is most probably associated with a
tunneling recombination process between deeply trapped electrons and holes, where the presence
of deep trap states has been reported.13,41 We note that recombination in titania is primarily nonradiative, so differences in direct versus indirect bandgaps reported between anatase and rutile
are unlikely to impact upon these charge carrier dynamics.
Hole scavenging by methanol
Methanol (and other alcohols such as isopropanol and glycerol) did not show any effect on the
TAS signals in dense samples, indicating that photo-generated holes in these dense films are
mostly unable to access the film surface (Figure S7). This correlates with the poor photocatalytic
activity observed for these dense films. This highlights the importance of photocatalyst surface
area in achieving a high hole scavenging efficiency – demonstrating how nano-structure in TiO2
is essential if bulk recombination is to be reduced.
The ability to generate long lived electrons by methanol hole scavenging was far more
pronounced in mesoporous anatase than mesoporous rutile (Figures 5 & 6). Holes in anatase
were completely scavenged by methanol on the pre-μs timescale, consistent with previous fs-
19
TAS studies that showing 80 % of holes were scavenged in 1 ns.16 In the absence of any
reduction targets, this results in the efficient generation of long lived electrons (~ 0.7 s). In
contrast, hole scavenging by methanol in mesoporous rutile appears to be much less effective.
The addition of methanol results in somewhat less efficient early time hole scavenging – with ~
2/3 of rutile holes being scavenged before 10 µs. However, more significantly, this hole
scavenging by methanol is much less effective at generating long lived electrons. In mesoporous
anatase, an almost threefold increase in electron population was observed from 10 µs with a
dramatic increase in the decay half-time from ~ 100 µs to ~ 0.7 s. However, in mesoporous
rutile, a mere 25 % increase in electron population was observed from 10 µs with only a
marginal increase in decay half-time from the ms timescale onwards. This strongly suggests that
methanol oxidation by rutile holes is at least partially reversible, such that it is relatively
ineffective at preventing electron losses. This inability of methanol (and other alcohols, see
supporting information) to suppress electron-hole recombination in rutile compared with anatase
is consistent with rutile’s lower photocatalytic activity as assayed in our intelligent ink study.
Our observation of less effective hole scavenging on rutile is in line with recent studies of
anatase and rutile single crystal surfaces, where anatase was consistently more active than rutile
in the photo-oxidation of methanol.42 The photocatalytic oxidiation of methanol by anatase has
been well studied.43 It is suggested the C-H bond of methanol is first cleaved by a surface hole
forming the ●CH2OH radical. Contrary to anatase, the methanol must first dissociate on the rutile
surface to form absorbed methoxy groups before C-H bond cleavage can occur,44 which is
perhaps why slower / more reversible scavenging kinetics are observed. This difference in hole
scavenging behavior was in contrast to the reactions of mesoporous anatase and rutile with the
electron scavenger AgNO3, where efficient and highly irreversible electron scavenging was
20
observed in both materials (Figures 3 and S10). This was a strong indication that the differences
in hole scavenging was not due to differences in porosity/ surface area and also showed how
photo-generated electrons could reach the surface of both mesoporous materials in high yield.
Correlation with photocatalytic activity.
We now turn to correlating the observation charge carrier dynamics with the efficiency of
photocatalytic function, considering the organic photo-degradation of intelligent ink. In
mesoporous anatase, rapid and highly irreversible hole scavenging by alcohol prolongs the
electron lifetime to ~ 0.7 s and thus strongly suppresses electron-hole recombination. These longlived electrons reduce the dye, resazurin, with a time constant of 3 ms, ~ 100 times faster than
competing decay pathways, leading to highly efficient photocatalysis reaction (QY = 62 %), as
illustrated in Scheme 2a. In contrast, the low photocatalytic activity of mesoporous rutile (QY =
1.9 %) results from less efficient and more reversible hole scavenging. As a result, hole
scavenging results in only a modest increase in electron lifetime, such that these electrons are
less able to reduce resazurin, as illustrated in Scheme 2b.
Scheme 2. Summary of the timescales of the processes in the photocatalysis of intelligent ink
with (a) mesoporous anatase and (b) mesoporous rutile; t1/2 half-times were measured from 10 μs
at ~ 300 μJ.cm-1. pulse-1. Dotted-lines represent the slowest transfer process in each system.
21
For both mesoporous anatase and rutile, direct electron transfer to our dye reduction target was
not observed in the absence of hole scavenger due to the kinetics of this dye reduction being
slower than bulk electron/ hole recombination. We note this inability to achieve dye reduction in
the absence of hole scavenger does not result from the inability of photo-generated electrons to
access the TiO2 surface, as efficient electron scavenging was observed in both materials with
AgNO3 (Figure S10). Rather it appears to result from the slow kinetics of electron transfer from
the TiO2 surface to the dye. We thus attribute the highly efficient and irreversible hole
scavenging of alcohols by mesoporous anatase as the primary reason for its enhanced
photocatalytic activity. The lower photocatalytic activity of dense rather than mesoporous TiO 2
films (Figure 2) also appears to result from inefficient hole scavenging, attributed to bulk
electron-hole recombination competing with hole transfer to the TiO2 surface.
The key impact of the efficiency of hole scavenging on photocatalytic function may be
understood in terms of the energetics of the TiO2 valence and conduction bands. TiO2 conduction
band electrons are not strongly reducing, such that reduction of organic materials (i.e. resazurin
dye or water) is kinetically slow. In contrast, TiO2 valence band holes are highly oxidizing,
enabling very rapid oxidation of suitable organic targets such as alcohols.
CONCLUSION
Mesoporous anatase was ~ 30 times more efficient than mesoporous rutile in the photocatalysis
of the ‘intelligent ink’ model system. Independent of phase, significantly lower levels of
intelligent ink photocatalysis were observed in dense films compared with mesoporous
structures. This was attributed to a high ratio of photo-generated charges residing in the material
22
bulk and not taking part in photocatalysis at the material surface – thus demonstrating the need
for nano-structuring TiO2 to achieve a higher efficiency of photocatalysis.
In the presence of alcohols, photo-generated holes on mesoporous anatase were scavenged
faster and more irreversibly than mesoporous rutile. As such, the addition of methanol is
substantially more effective at generating long lived electrons on anatase compared to rutile. In
the presence of intelligent ink, efficient electron transfer from anatase to the redox dye resazurin
was observed on the millisecond timescale. It is concluded that the poorer photocatalytic activity
of rutile compared to anatase results not from differences in non-geminate recombination, nor the
efficiency of electron access to the TiO2 surface, but rather from the poorer ability of rutile holes
to drive the efficient and irreversible oxidation of alcohol scavengers. In the case of mesoporous
rutile, hole scavenging was reversible on a time-scale faster than the photocatalysis of the dye,
resulting in a low efficiency. This is of high significance to those studying colloidal TiO2
photocatalysts, where alcohol hole scavengers are often employed to drive photocatalytic
processes.
Supporting Information. Lamp excitation source emission spectra, XRD patterns, UV-visible
absorption spectra, BET analysis, SEM images and additional transient absorption spectra
provided. This material is available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Authors
23
*[email protected] and [email protected]
Funding Sources
ERC Intersolar (291482)
Ramsay Memorial Fellowships Trust
ACKNOWLEDGMENTS
AK wishes to thank the Ramsay Memorial Fellowships Trust for funding his fellowship. JRD
wishes to thank the ERC project Intersolar (291482).
ABBREVIATIONS
TAS, transient absorption spectroscopy; CVD, chemical vapor deposition; QY, quantum yield;
SEM, scanning electron microscopy; BET, Brunauer–Emmett–Teller; BJH, Barret-JoynerHalenda.
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